The present invention relates to a copying machine provided detachably with a plurality of developing units which contain different colors of toners.
Recently there has been an increasing demand for color copies and in order to meet this demand there have been provided image forming apparatus which permit copying in colors in addition to black copy. In the case of a small-sized image forming apparatus, the number of developing units capable of being loaded into the apparatus is generally up to two due to a structural restriction. Color images formed using desired colors of toners can be obtained by changing developing units.
Thus, an image forming apparatus of a structure which permit easy replacement of developing units is desirable in order to obtain desired colors of images by changing developing units.
On the other hand, when an image of a color different from the colors of toners contained in the loaded developing units is to be obtained, it is possible that a developing unit which is not in use will be detached by mistake during the image forming operation of the image forming apparatus. This results in incidence of extraneous light from this portion onto the photosensitive drum and so a latent image which has been formed on the drum is erased, discharging a white copy sheet.
According to conventional methods proposed for solving this problem, a door is attached to the developing unit loading portion so that the replacement of a developing unit requires the door to be opened, or there is provided a safety device involving a switch which operates upon opening of the said door so that the image forming operation is discontinued when the door is opened to change a developing unit. As a result, however, the replacement of a developing unit becomes more troublesome.
On the other hand, if copying is made in a large volume from a high image density original using a copying machine, the toner in the developer becomes scarce, resulting in that the image density reproduced of a paper becomes lower. To avoid this inconvenience there has been developed a copying machine capable of supplying toner quickly to a developing device from a toner supply bottle upon actuation of a switch which is done by the operator when the operator judges it necessary to supply toner.
In the case of providing toner supply means for plural developing units, there occur the following problems unlike the case where only one developing unit is provided. For example, when the start of color copy is commanded during quick supply of toner to a color developing unit, it is possible to perform the copying operation under quick supply of toner without causing any special problem. On the other hand, if there is issued a copy start command for a developing unit different from the developing unit being supplied with toner, that is, in the above example, if the copying mode is changed over to a black copy mode and a copy start command is issued, the black developing unit starts operation. In this case, it is necessary to provide indicating means for indicating whether the selected developing unit is of black or color and to which developing unit the toner has been supplied. Otherwise, it would be impossible to grasp the condition, thus causing malfunction. Even if such indication is made, there still remains the possibility of malfunction based on erroneous judgement because of parallel arrangement of the indication of the selected developing unit and that of the developing unit under quick supply of toner. Further, in the color developing unit which has been supplied with toner, the developer agitating operation has also been performed simultaneously with supply of toner, but the agitation for the developer in the color developing unit is discontinued because the power transmission from the driving motor is changed over to the black developing unit. That is, carrier and toner in the developer have not been intimately mixed yet, thus affecting the image formation. Although this problem can be overcome by transmitting power to both developing units, there arises the necessity of providing a driving motor for each developing unit or providing a large-sized driving motor. Such a construction is not adoptable in small-sized apparatus.
Further, in the case of a developing unit provided with toner supply means, it is known to detect and display a decrease in the amount of toner remaining in the toner supply means to a level below a predetermined level.
For indicating the residual amount of toner in toner supply means or indicating that the toner supply means is empty, in a copying machine having plural developing units, it is the simplest method to provide a dedicated indicating element for each of the developing units. However, when these indicating elements are provided on an operating panel, a large number of indicating elements are arranged on the same panel, resulting in that the confirming operation becomes complicated. If such toner residual amount indicating elements are transferred from the upper surface of the operating panel to any other portion of the apparatus, there arises the possibility of forgetting the confirmation of indication. Anyhow, the increase in the number of indicating elements causes increase of the cost.
In the case where only one toner residual quantity indicating element is provided for a plurality of developing units and it with -- gives indication when a desired developing unit is selected and the toner supply means attached thereto is empty, it is impossible to know that there is no toner until the selection of the developing unit.
Further, the toner supplying operation, which is performed by the replacement of a toner supply bottle or by any other suitable means, is a relatively time-consuming operation, so it has been desired to provide means capable of confirming in advance that there is no toner remaining in the bottle.